Testing the direct effect of CO 2 concentration on a bloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments

We studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (~11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (~190 parts per million by volume (ppmV) CO 2 ), present (~410 ppmV CO 2 ), and high (~710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19‐day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 ‐related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi , the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.